Replacing Pressure Gauge on WB Greenstar 28i?

I've just had a repair and service done on my Woster bosch greenstar 28i and every thing is now working ok. Appart from pressure guage is no longer working. I questioned the engineer just as he was leaving and he made out he hadn't noticed but evey thing was fine and he must of caught and nipped the pipe by accident and he would return and fit a new manometer in the next week. Maybe I shouldn't have paid him the full amount but i suspect he's not going to come back to fix it.
My question is if he doesn't show can I buy a replacement (£20 ebay) and swap it over myself. I'm assuming you have to drain and refill the system. Is this something I could do myself. How would i go about this.

ANON1
September 2010